Passive imaging is a key technique for target detection, discrimination, and classification. Advances in imaging, spectral analysis, and active ranging now exploit larger regions of the electromagnetic spectrum from the ultraviolet (“UV”) region to the very long wave infrared (“VLWIR”) region to identify and separate targets from backgrounds and decoys. Existing imaging systems may detect electromagnetic wavelength, phase front, and time-of-flight to extract signatures of targets. However, one electromagnetic wave physical property that is not being fully exploited using existing technology is wave polarization state. Polarimetry, which is the measurement and interpretation of the polarization of electromagnetic waves, has potential applications such as target discrimination, buried mine detection, hidden object detection, measurement of sugar content in foods, purity measurement of pharmaceutical materials, and measurement of blood glucose.
Polarization of an electromagnetic wave can be characterized using a polarimetric imager. One existing polarimetric imager uses a polarizing beam-splitter and two cameras. Such imager suffers from the expense and space required to provide two cameras and the difficulty in spatially and temporally registering or aligning images from the two cameras. This imager also does not measure circular polarization.
Another existing polarimetric imager uses a rotating linear polarizer to acquire successive images at different polarizations. However, this imager requires significant time to scan the various polarizations, which may result in generation of artifacts due to motion of the target or camera, even from a leaf blowing in the wind.
A more recently developed polarimetric imager uses a “micropolarizer array”. This imager measures four separate linear polarizations (0°, 90°, +45°, −45° on adjacent pixels, which are not coincident, and also reduces spatial resolution by four to one. Additionally, it may be difficult to align this imager with its focal plane array. Furthermore, this imager does not measure circular polarization.